1. Field of the Invention
The present invention relates to a laser control method and a laser control circuit which control an applied laser output when laser light is applied from a semiconductor laser element to an irradiated body.
2. Description of the Related Art
In the past, a semiconductor laser has been used in a data recording device which records data on a recordable optical disc and a recording device which records data in the process of producing a master optical disc (stamper) (so-called cutting device).
The master optical disc is produced as follows, for example. A resist film is formed on a glass substrate. Thereafter, the resist film is irradiated with laser light emitted from a semiconductor laser in accordance with ON-OFF control. Thereby, a latent image according to a data pattern is thermally recorded on the resist film. Then, the resist film is subjected to a development process, and a portion irradiated with the laser light is removed therefrom. Thereafter, the resist film is metal-plated, e.g., nickel-plated. Then, a nickel-plated layer is peeled off from the resist layer to form a stamper, which is a master optical disc.
In the above-described data recording device, which records data on an optical disc, and a recording device for a master optical disc, which uses a PTM (Phase Transition Mastering) microfabrication technology, a write strategy is commonly used as a method of controlling recording laser light. There are a variety of methods for the write strategy. In the write strategy, the light emission waveform characteristic of the laser light is important. The recording characteristic is deteriorated due to waveform degradation such as rounding of a light emission waveform with respect to an input signal. That is, the light emission waveform is desired to have a rectangular wave 101 illustrated in FIG. 10A. With a rounded waveform 102 illustrated in FIG. 10B, the recording characteristic is deteriorated. The waveform degradation tends to occur particularly in high-speed recording or the like. To ensure the recording characteristic, a favorable signal transmission characteristic is desired in a range including a higher frequency band.
FIG. 11 illustrates an example of an existing laser control circuit. The laser control circuit 103 is configured such that a light receiving element 105 receives a part L1 of laser light emitted from a semiconductor laser element 104, and that a feedback is applied to a signal output from the light receiving element 105 to electrically control the laser output to continue to be constant. This type of laser control is generally called Auto Power Control (APC).
For example, a recording device used in the production of a master optical disc is provided with an optical pickup for recording data, and laser light from a semiconductor laser element is applied to a resist film through respective optical elements. The laser control circuit 103 of FIG. 11 is configured as a feedback circuit including a photodiode (PD), which is the light receiving element 105 (hereinafter referred to as the photodiode 105) for receiving the part L1 of the laser light emitted from the semiconductor laser element (LD) 104 of the optical pickup. That is, the laser control circuit 103 is configured to include the photodiode 105, an adder 106, an operational amplifier 107, and a transistor 108. An output signal of the photodiode 105 and a bias setting voltage are input to the adder 106, and an output of the adder 106 is input to an inverting input terminal (−) of the operational amplifier 107. The output signal of the photodiode 105 corresponds to an output voltage according to the laser output (light intensity) from the semiconductor laser element 104. A non-inverting input terminal (+) of the operational amplifier 107 receives an input of an input signal, i.e., a write signal (pulse signal) corresponding to data to be recorded. The output of the operational amplifier 107 is input to the base of the transistor 108. The collector of the transistor 108 is connected to a power supply, and the emitter of the transistor 108 is connected to the anode of the semiconductor laser element 104.
In the laser control circuit 103, the laser characteristic of the semiconductor laser element 104 fluctuates due to long-time use. That is, in the laser characteristic set to obtain a specified laser output with respect to a specified write signal input to the non-inverting input terminal (+) of the operational amplifier 107, the bias setting voltage and the gain setting voltage fluctuate. If the laser output is reduced due to the fluctuation of the laser characteristic, the output signal of the photodiode 105 is reduced, and the input from the adder 106 to the inverting input terminal (−) of the operational amplifier 107 is reduced. As a result, the output of the operational amplifier 107 is increased, and the current flowing through the semiconductor laser element 104 is increased. Thereby, the laser output is maintained to be constant.
Even if the laser control circuit 103 is formed by the fastest device available at present, the relationship between the frequency of the write signal and the laser output is as illustrated in FIG. 12, wherein the output is reduced in an approximate range of 50 MHz to 100 MHz.
As illustrated in FIG. 13, as a measure to address the above issue, a laser control circuit 110 has been used which corresponds to the laser control circuit 103 of FIG. 11 inserted with a compensation circuit 113 for compensating a high-frequency range. The laser control circuit 110 is provided with the compensation circuit 113, i.e., a so-called high-frequency feedback circuit, which includes a series circuit formed by a capacitor (C) 111 and a resistor (R) 112 and inserted between the output of the operational amplifier 107 and one of the input terminals of the adder 106 in FIG. 11.
In the laser control circuit 110, a low-frequency range is compensated by the feedback of the laser light, and a high-frequency range is compensated by the high-frequency feedback circuit formed by the compensation circuit 113. As illustrated in the frequency characteristic of FIG. 14, therefore, the laser output is expected to be stabilized in a range including the high-frequency range.
Japanese Unexamined Patent Application Publication No. 2004-186626 discloses a semiconductor laser drive device which controls and drives a semiconductor laser of a cutting device for a master optical disc.